This application is based upon Japanese Patent Application No. Hei. 11-206500 filed on Jul. 21, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to map display apparatuses, and particular to a map display apparatus for displaying a map based on map data that can correct a distortion of the map data, wherein the map is generated by assuming that an area surrounded by particular longitudes and particular latitudes is a rectangle or a square.
2. Related Art
Conventionally, a map display apparatus such as a vehicular navigation system generally includes a function for displaying a map including a present position and/or a target position (destination) assigned by a user. In general, map data used in such a system is generated by assuming that an area surrounded by particular longitudes and particular latitudes is a rectangle or a square. Specifically, the map data is administrated every predetermined longitude width and every predetermined latitude width, and is normalized to the same size blocks.
In this case, since the data is normalized so that an actual length at a predetermined reference latitude point is accurate, a low latitude area positioned lower than the reference latitude point is normalized shorter than the actual length. On the other hand, a high latitude area positioned higher than the reference latitude point is normalized longer than the actual length. Therefore, a distortion becomes larger while moving away from the reference latitude. In order to solve the above problem, a map display method in which drawing coordinate data is converted by using a correction coefficient, wherein the correction coefficient is a fixed value that corresponds to a ratio of actual length of longitude width and latitude width of a block to be displayed. However, since this map display method converts the map data by using a constant value, the map around the Tokyo area may be accurately displayed without distortion. However, the map around Hokkaido may be distorted as a result of an inaccurate aspect ratio.
A countermeasure to solve this problem is disclosed in JP A 8-328462. According to this countermeasure, the correction coefficient is determined by calculating an approximated rectangle based on the latitude of a center point in a map to be displayed during displaying. However, when the direction of travel on the map is changeable, it needs a multiplication process to calculate a rotation angle of the map with reference to a reference direction and the correction coefficient. Specifically, since data for all points of the map to be displayed are converted every time the map is displayed, it needs two calculation processes including an enlarge/reduce process of the map by using the multiplication of the correction coefficient, and a rotation process of the map by using the multiplication of the rotation angle. Furthermore, since these processes are necessary for all coordinates within the map to be displayed every time the map is drawn, it takes a long time to draw the map.
This invention has been conceived in view of the background thus far described, and its first object is to adequately correct distortion of a map by restricting a decrease of a map drawing speed.
Its second object is to provide a map display apparatus for displaying map data generated by assuming that an area surrounded by particular longitudes and particular latitudes is a rectangle or a square, and for adequately correcting distortion of map with restricting a decrease of a map drawing speed.
According to a first aspect of the present invention, distortion correction coefficients are previously stored in a distortion correction coefficient storage portion by a unit of a predetermined latitude width. A display control portion reads out a particular distortion correction coefficient corresponding a latitude of a particular point in a target area, and generates a map by converting a previously stored map data using the particular distortion correction coefficient.
Since the present invention uses the previously stored distortion correction coefficients, it becomes unnecessary to calculate the distortion correction coefficients every time a map is drawn. As a result, it can adequately correct distortion of map with restricting a decrease of a map drawing speed.
According to a second aspect of the present invention, in addition to the first aspect of the present invention, a coordinate conversion correction coefficient storage portion stores coordinate conversion correction coefficients by a unit of a predetermined latitude width with respect to each rotation correction coefficient corresponding to each rotation angle. The coordinate conversion correction coefficients are determined by multiplying the rotation correction coefficient corresponding to each rotation angle by the distortion correction coefficient. Therefore, when the coordinate conversion is performed by using the coordinate conversion correction coefficient, a multiplication process of the correction coefficient can be substantially performed at the same time. Therefore, it can eliminate a process for multiplying the correction coefficient of the rotation angle by the distortion correction coefficient while drawing the map.
According to a third aspect of the present invention, a present position detection portion detects a present position. A distortion correction coefficient determination portion determines a distortion correction coefficient at a latitude of the present position detected by the present position detection portion after power on. A distortion correction coefficient storage portion stores distortion correction coefficient determined by the distortion correction coefficient determination portion. A display control portion converts the map data using the particular distortion correction coefficient stored in the distortion correction coefficient storage portion. As a result, an amount of data to be stored can be reduced.